1.) Symptoms: Cory's tail fin is almost entirely gone. A few short spines are left here and there but it's mostly down to the body. A third or so of his body above the tail is whitish. I would have thought he was dead when I saw him from the skin color but he started swimming. He's still pretty fast! Last time I know I saw him for sure with his tail intact was Thursday. I'm wondering if fin rot or stress could decimate the fin so fast or if he's been nibbled on or injured by the filter or something?

6.) Recent changes: Platies added to tank 18 days ago along with 1 oz BioSpira. Ick spots on 1 within 24hrs. Heat/darkness treatment for 10 days, no medication. All platies survived and seem perfectly healthy, very active. Cories added to tank 2 days ago (Jan 4) w/o BioSpira. Except for the tailless one, all cories healthy and active. I can't be sure it's the same cory, but when I bought them, there was one in the bag who wasn't as active as the others. I asked about that one and we all peered into the bag for a while, and he seemed intact and fine at the time. The LFS employee said to bring him back if he didn't make it.

7.) Food: Tetra-min Select-A-Meal once or twice a day, 1 Hikari sinking wafer each day since cories added.

That is fin rot which is what killed 80% of my tank off, I tried tank buddies (fungus medication) that didn't work, then I tried some others and none of them worked, and then I tried MelaFix and it is now curing my 2 remaining silver dollars.It is pritty cheap to buy and is all natural so you can give it with antibiotics and it will not kill the fish. I would try and do a gravel clean asap and get some of the excess garbage gone to lower the nitrate levels alittle, fin rot it very frustrating because it can also be a secondary disease so thats why I would start off with MelaFix and then look out for other signs of something like a Parasite.

Also when you leave it until it's down to the body it's pritty much to late because its going to attack the body and kill the fish, you need to catch it when it's only attacking the fins.

My first thought was also Fin Rot, but I had no idea it could work that fast. This fish was intact less than 48 hours ago. I just did some more pointed websearching and found out that fungal finrot (as opposed to bacterial finrot) can take a whole tail overnight. WOW.

I can be at the LFS tomorrow when they open to buy Melafix and Fungus Clear? Fungus Eliminator? Any other suggestions?

My first thought was also Fin Rot, but I had no idea it could work that fast. This fish was intact less than 48 hours ago. I just did some more pointed websearching and found out that fungal finrot (as opposed to bacterial finrot) can take a whole tail overnight. WOW.

I can be at the LFS tomorrow when they open to buy Melafix and Fungus Clear? Fungus Eliminator? Any other suggestions?

I've used Fungus Clear from Jungle Labs and it works very well along with Melafix it will clear up VERY quickly. If they don't have Melafix the next best or the same thing would be Pimafix. You may not save the really bad one but it is a very good idea to treat just the same to make sure the others do not get it.